Thermodynamic properties of dense gaseous and liquid argon are calculated using perturbation theory based on the hard-sphere potential, together with an accurately determined pair potential function and the triple-dipole dispersion three-body interaction. The first-order contribution of the three-body interaction is calculated both by a Monte Carlo method and by the use of the superposition approximation for the three-body distribution function, with good agreement. Monte Carlo estimates are also found for second-order contributions of the three-body interactions, which prove to be small. Agreement with experiment is excellent at high temperatures and good at low temperatures. Slight discrepancies at low temperatures are probably due partly to the use of the ‘local compressibility’ approximation and perhaps partly to slight uncertainty in the pair potential in the neighbourhood of its zero. © 1969 Taylor and Francis Group, LLC.